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United States Patent |
5,132,728
|
Suzaki
,   et al.
|
July 21, 1992
|
Drum driving unit of electrophotography printer
Abstract
An electrophotography printer includes a drum formed of a non-magnetic
cylindrical body having an outer circumferential surface to which a
photosensitive material is applied, end brackets attached to opposite ends
of the drum, and a drum shaft for transmitting a driving torque to the
drum. A connection structure between the drum and the drum shaft includes
end bracket shafts provided on the end brackets, a hole formed in the drum
shaft so as to be capable of engaging with one of the end bracket shafts,
parallel pins provided on the end bracket shafts so as to be perpendicular
to the axes thereof, and at least one taper groove formed in an end
portion of the drum shaft. When the drum is housed, one of the parallel
pins and the taper groove engage with each other while the whole of the
drum is pressed toward the drum shaft by an elastic member, thereby
reducing the non-uniformity of the rotation of the drum.
Inventors:
|
Suzaki; Masafumi (Hitachi, JP);
Kikuchi; Tsugio (Ibaraki, JP);
Nagano; Yousuke (Hitachi, JP);
Homma; Yoshifumi (Hitachi, JP);
Ebata; Kenichi (Hitachi, JP)
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Assignee:
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Hitachi, Ltd. (Tokyo, JP)
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Appl. No.:
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521524 |
Filed:
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May 10, 1990 |
Foreign Application Priority Data
Current U.S. Class: |
399/167; 403/355; 492/15 |
Intern'l Class: |
G03G 015/00 |
Field of Search: |
355/200,210,211,212
403/354,355,378,379
29/115,116.1,122,123
|
References Cited
U.S. Patent Documents
2300514 | Nov., 1942 | Lewis | 403/354.
|
4425036 | Jan., 1984 | Kameyama et al. | 355/200.
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Foreign Patent Documents |
193931 | Nov., 1983 | JP.
| |
96317 | Apr., 1988 | JP.
| |
75274 | May., 1989 | JP.
| |
Primary Examiner: Moses; R. L.
Assistant Examiner: Horgan; Christopher
Attorney, Agent or Firm: Antonelli, Terry Stout & Kraus
Claims
What is claimed is:
1. A drum driving unit of an electrophotography printer including a drum
formed of a non-magnetic cylindrical body having an outer circumferential
surface to which a photosensitive material is applied, end brackets
attached to opposite ends of said drum, and a drum shaft for transmitting
a driving torque to said drum, said drum driving unit comprising:
end bracket shafts provided on said end brackets;
a hole formed in said drum shaft, said hole being capable of engaging with
one of said end bracket shafts;
parallel pins provided on said end bracket shafts so as to be perpendicular
to the axes thereof; and
at least one tapered groove formed in an end portion of said drum shaft;
wherein when said drum is housed, one of said parallel pins and said taper
groove engage with each other while the whole of said drum is pressed
toward said drum shaft by an elastic member, and
wherein an angle .theta. of said taper groove is selected so that if an
elasticity force applied to said drum is W.sub.s, the driving load torque
of said drum is T and the average diameter of said drum shaft is D,
2T/D.multidot.tan.theta.<W.sub.s can be established, and the elasticity
force W.sub.s ranges to at most 3 kg.
Description
BACKGROUND OF THE INVENTION
This invention relates to an electrophotography printer such as a laser
beam printer and, more particularly, to a drum driving unit of this kind
of printer.
A drum driving unit of a conventional electrophotography printer will be
described below with reference to FIG. 2. An end bracket 2 is attached to
an end of a drum 1 constituted by a non-magnetic cylindrical body having
an outer circumferential surface to which a photosensitive material is
applied. An end bracket shaft 3 is attached to the end bracket 2 coaxially
with the same. A spring type one-way clutch 5 is coaxially incorporated in
a drum shaft 4 for transmitting a driving torque to the drum 1. The drum 1
is an expendable member and needs to be changed periodically. To change
the drum 1, a drum assembly 6 integrally formed the drum 1, the end
bracket 2 and the end bracket shaft 3 is changed. The end bracket shaft 3
of a new drum 1 is brought into engagement with the one-way clutch 5. In
ordinary electrophotography printers, the drum 1 is rotated in only one
direction. The arrangement may therefore be such that the one-way clutch 5
is locked with respect to the direction of rotation of the drum 1 to
enable the torque of the drum shaft 4 to be transmitted to the drum
assembly 6. At the time of of the exchange, the attachment position of the
drum 1 in the rotational direction is not particularly limited, and it is
sufficient to establish engagement between the end bracket shaft 3 and the
one-way clutch 5 incorporated in the drum shaft 4.
The above-described conventional art entails a problem described below.
FIG. 3 shows the results of an actual test which was conducted in such a
manner that an optical encoder was attached to the drum of a conventional
type of printer to detect the rotational speed of the drum and, hence, to
measure the variation in the rotational speed (unevenness) with respect to
a reference speed during one revolution of the drum. In FIG. 3, segments B
which indicate abnormally large speed variations are clearly seen between
segments A which indicate speeds in a normal range. Variations in the drum
speed are caused by the variation in the force of friction between the
drum and a blade cleaner for removing surplus part of the ink applied to
the drum, but deteriorations of the image qualities caused by the
influence of these variations are small. Therefore the present invention
is not intended to remove small speed variations such as those indicated
by the segments A. FIG. 4 schematically shows features of an actual image
sample printed on the condition shown in FIG. 3. This actual image sample
is called a half tone image formed by drawing lateral lines with small
regular pitches to obtain a half tone. Since the recent market trend is
being shifted from setting importance on characters to setting importance
on graphics, the subject of how to output half tone images having improved
qualities is important. In the conventional printers, however, the
printing pitches in the direction of rotation of the drum is made uneven
due to unevenness of the rotation of the drum 1 (speed variations),
resulting in the formation of a light-shade striped pattern, such as that
shown in FIG. 4. This phenomenon considerably deteriorates the image
qualities. It is therefore very important to prevent this phenomenon.
The reason for the occurrence of such large variations in the rotational
speed is that the one-way clutch is repeatedly locked and released
irregularly by small vibrations and variations in the load, since the
one-way clutch has a restraining force with respect to rotation in only
one direction and has no restraining force in the opposite direction.
Japanese Utility Model Unexamined Publication No. 1-75274 discloses a
construction in which the entire drum is attached with a pressing force of
a spring. However, it is not possible to attain an object of the present
invention, i.e., the object of preventing unevenness of the drum rotation
and, hence, light-shade unevenness of a half tone by only simply pressing
the drum.
Japanese Patent Unexamined Publication No. 58-193931 discloses a method
relating to a shaft joint, in which a member constituted by an elastic
body having a taper groove is attached to a groove portion of a shaft, and
this groove portion is engaged with a spring pin. For driving a laser
printer drum, however, it is not suitable to apply the construction in
which an elastic member is used for a shaft joint, because the elastic
member is elastically deformed with changes in the load to cause
vibrations in the axial direction, resulting in light-shade unevenness in
the printed image.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a drum
driving unit for use in an electrophotography printer in which a structure
including the connection between a drum which is an expendable member and
which needs to be changed and a drum shaft for driving torque transmission
which must therefore be separated from the drum is optimized to reduce the
non-uniformity of drum rotation and, hence, to obtain good graphics while
enabling the drum to be changed in a simple manner.
To achieve this object, according to the present invention, a drum driving
unit is provided including a hole formed in the drum shaft, with end
bracket shafts, being formed on the end brackets so as to be capable of
engaging with the hole, and with parallel pins being provided on the end
bracket shafts so as to be perpendicular to the axes thereof. A taper
groove is formed in an end portion of the drum shaft, and when the drum is
housed, one of the parallel pins and the taper groove engage with each
other while the entire drum is pressed toward the drum shaft by an elastic
member.
According to further features of the present invention, a drum driving unit
is provided including a hole formed in at least one of the end brackets,
with an engagement shaft portion being formed on the drum shaft so as to
be capable of engaging with the hole, and with a parallel pin being
provided on the engagement shaft portion perpendicularly to the same. A
taper groove is formed in an end portion of the end bracket where the hole
is formed, and when the drum is housed, the parallel pin and the taper
groove engage with each other while the whole of the drum is pressed
toward the drum shaft by an elastic member.
In the construction of the present invention including the hole in the drum
shaft and the end bracket shaft provided on the corresponding end bracket,
the axes of the drum shaft and the end bracket shaft are aligned with each
other and one end of the drum assembly is supported by the drum shaft. The
construction including the parallel pins perpendicular to the end bracket
shafts and the taper groove formed in the end portion of the drum shaft,
in which the parallel pin and the taper groove engage with each other when
the drum is housed ensures that a driving torque of the drum shaft is
transmitted to the drum assembly. The construction in which the entire
drum is pressed toward the drum shaft by an elastic member such as a
spring ensures that the parallel pin is pressed against taper surfaces of
the taper groove so that the engagement between the drum shaft and the end
bracket shaft is free from any play. Consequently, the drum shaft and the
drum are connected like an integral member, thereby eliminating the
possibility of occurrence of large variations in the rotational speed of
the drum (unevenness of rotation). Even if, when the drum is set in the
printer, the parallel pin is positioned on the drum shaft end without
being engaged with the taper groove, depending upon its position in the
direction of rotation of the drum, the pin can be easily brought into
engagement with the taper groove as the drum shaft rotates during a
warm-up of the printer, because a thrust force based on the force of the
elastic member, i.e. spring, is applied to the entire drum.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of drum engagement portions in accordance with
an embodiment of the present invention;
FIG. 2 is a perspective view of drum engagement portions of a conventional
drum driving unit;
FIG. 3 is a graph showing data on the measurement of the variation in the
rotational speed of the conventional drum;
FIG. 4 is a schematic diagram of an actual image sample obtained by a
printer using the conventional drum driving unit;
FIG. 5 is a cross-sectional view of the construction of a laser beam
printer to which the present invention is applied;
FIG. 6 is a cross-sectional view of a unit for rotating the drum in
accordance with the present invention;
FIG. 7 is a graph showing data on the measurement of the variation in the
rotational speed of the drum in accordance with the present invention;
FIG. 8 is a schematic diagram of an actual image sample obtained by the
printer using the drum driving unit of the present invention;
FIG. 9 is a perspective view of an example of a modification of the drum
engagement portions in accordance with the present invention; and
FIG. 10 is a diagram of the setting of the angle of the taper groove
portions.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An embodiment of the present invention will be described below with
reference to FIGS. 1, 5, and 6. As shown in FIG. 5, a laser beam printer
in accordance with the present invention includes an electrifier 7, an
optical unit 8, a development device 9, a transfer/discharge device 10, a
cleaning unit 11 and an erasing lamp 12 disposed around a drum 6 to which
a photosensitive material is applied. These components are arranged to
effect an electrophotography process. A printing sheet cassette 13, a
fixation unit 14 and a printing sheet discharge tray 15 are also provided.
As shown in FIG. 6, a drum driving mechanism in accordance with the present
invention includes a drum assembly 18 disposed between a pair of body side
plates 16 and 17. The drum assembly 18 has a drum 19 in the form of a
hollow cylinder formed of a non-magnetic material such as aluminum and
having an outer circumferential surface to which a photosensitive material
is applied, end brackets 20 respectively fixed by press-fitting to
opposite ends of the drum 19, end bracket shafts 21 respectively fixed by
press-fitting to the end brackets 20, and parallel pins 22 respectively
fitted by press-fitting to the end bracket shafts 21. The drum assembly 18
is symmetrical with respect to a plane transversely bisecting the drum.
One of the end bracket shafts 21 is engaged with and is supported on a drum
shaft 23 for transmitting a driving torque to the drum assembly 18 in a
manner described more fully hereinbelow with reference to FIG. 1.
The other end bracket shaft 21 is supported on the printer body by a drum
support plate assembly 29 consisting of a drum support plate 24, a bracket
26 and a ball bearing 28. The drum support plate 24 is detachably attached
to the body side plate 17, the bracket 26 is attached to the drum support
plate 24 with screws 25, and the bearing 28 is fitted in the bracket 26 so
as to be movable in the axial direction. The drum assembly 18 can be
interchanged in such a manner that the entire drum support plate assembly
29 is detached and the drum assembly 18 is thereafter drawn out in the
direction of the arrow A.
A cylindrical coil spring 30 is provided in the bracket 26 to constantly
urge by the spring force the entire drum assembly 18 in the direction of
the arrow B through the ball bearing 28 and the parallel pin 22.
A construction for transmitting the driving torque of the drum driving
motor to the drum shaft 23 will be described below.
A driving motor unit 44 has an opposed flat plate type dc motor. A magnet
46 in the form of a disk is mounted on a flat plate 45. The flat plate 45
is rotated by energizing a coil 47 disposed so as to face the magnet 46.
The driving torque of this plate is transmitted to the drum shaft through
a motor pinion 48 integrally attached to the flat plate 45, a first stage
gear 49, a second stage pinion 50, a second stage gear 51, a first pulley
52, a timing belt 53, and a second pulley 54.
As shown in FIG. 1, the end brackets 20, the end bracket shafts 21 and the
parallel pins 22 are integrally attached to the opposite ends of the drum
19 so as to be symmetrical with respect to a plane transversely bisecting
the drum 19. The drum shaft 23 has a hole 23a in which the end bracket
shaft 21 is inserted (fitted). The hole 23a is formed with a slight
tolerance defined between its diameter and the outside diameter of the end
bracket shaft 21 so as to be capable of being fitted around the same
without any substantial play in the radial direction at the time of
attachment of the drum as well as to be capable of being disengaged from
the end bracket shaft. Taper grooves 23b are formed in the shaft 23 at an
end of the same so as to be capable of receiving the parallel pin 22 when
the end bracket shaft 21 is inserted into the hole 23a. The groove 23b are
provided with taper surfaces in order to prevent engagement play between
the parallel pin 22 and the grooves and to thereby prevent variations in
the rotational speed (unevenness of rotation) of the drum during rotation
of the same. That is, since the entire drum 18 is urged in the direction
of the arrow B, the outer circumferential surfaces of the parallel pin 22
are brought into engagement with the taper portions of the taper grooves
23b without permitting any engagement play. Even if, at the time of
setting of the drum 18, the parallel pin 22 does not engage with the taper
grooves 23b and is positioned on a drum shaft end 23c, the cylindrical
coil spring 30 is compressed to enable the drum assembly 18 to be housed
in the printer. Thereafter, as the drum shaft 23 is rotated during warm-up
of the printer (while the drum is stopped at the set position), the
parallel pin is moved to the position at which it can be engaged with the
grooves, thereby being automatically brought into engagement. Additional
taper grooves may be provided so that the grooves are arranged crisscross,
and further taper grooves may be provided.
FIG. 7 shows the results of a measurement of the variation in the
rotational speed of a printer drum actually constructed in accordance with
the present invention. (FIG. 7 corresponds to FIG. 3 showing the speed
variation in the conventional arrangement). As is apparent from FIG. 7,
the present invention is free from any substantially large irregular speed
variations, and the performance is good.
FIG. 8 schematically shows the results of printing of a half tone image
actually performed by using the printer in accordance with the present
invention. As is readily apparent from FIG. 8, no light-shape striped
pattern is exhibited and a high-quality image is obtained.
In the embodiment of FIG. 9, a fitting shaft portion 35a is provided on a
drum shaft 35, and a parallel pin 22 is press-fitted in the fitting shaft
portion 35a perpendicularly to the same. An end bracket 20 is press-fitted
in the drum 19. A boss 36 is integrally formed on the end bracket 20. The
boss portion 36 has a fitting hole 36a in which the fitting shaft 35a of
the drum shaft is fitted, and taper groove portions 36b. This arrangement
also ensures the same effects described hereinabove in connection with the
embodiment of FIGS. 1, 5 and 6.
The setting of the angle of the taper grooves will be described below with
reference to FIG. 10. If the force of the cylindrical coil spring for
urging the drum is W.sub.s, W.sub.s must be substantially smaller than an
ordinary manual pressing force, i.e., 3 kg or less. Assuming the load
torque applied to the drum be T and the average diameter of the drum shaft
(the mean of outside and inside diameters) be D as shown in FIG. 10, a
force of W=2T/D is applied to the parallel pin in a perpendicular
direction.
The parallel pin is engaged with the groove portion having an inclination
of an angle .theta.. A thrust force therefore acts to press the spring
backward, which force is expressed by F.sub.s
=Wtan.theta.=2T/D.multidot.tan.theta.. F.sub.s must be smaller than
W.sub.s in order that the parallel pin is firmly engaged with the taper
groove portions. Accordingly, W.sub.s .gtoreq.2T/D.multidot.tan.theta.must
be established. In this embodiment, T=6 kg-cm, D=2.2 cm and
.theta.=15.degree. are set.
In accordance with the present invention, as described above, the
engagement between the drum shaft and the drum is free from any play, and
the non-uniformity of the drum rotation is reduced. It is thereby possible
to prevent occurrence of light-shade unevenness in half tone images
experienced as a problem of electrophotogaphy, thus achieving an
improvement in image qualities. The present invention also makes it
possible to realize a printer capable of interchanging the drum by a
simple operation and thus improved in terms of maintenance facility.
Since the end brackets, the end bracket shafts and parallel pins can be
designed for common use in a symmetrical arrangement, the drum assembly
can be produced at a high efficiency, and the possibility of occurrence of
errors in assembly during production or maintenance operation is reduced,
thereby achieving improvements in producibility and assembly facility.
The provision of the parallel pins at the opposite ends of the drum makes
it possible to simultaneously realize three functions of transmitting a
torque, preventing play and applying a thrust force. It is therefore
possible to provide a drum driving unit having a simple construction and
improve in performance.
The bearing disposed opposite to the drum shaft can have both functions of
supporting the shaft and applying a thrust force, thereby being simplified
in structure. Also, the disassembly/assembly performance can be improved
while the desired low speed rotation precision is maintained, if the taper
groove angle .theta. is selected suitably. Since the taper grooves are
formed in a rigid body, there is no influence of torsional vibrations in
the rotational direction due to changes in the load, which problem is
encountered in the case of an elastic grooved member. Stable rotational
speed characteristics can therefore be obtained.
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